Method and apparatus for controlling back pressure

ABSTRACT

A back pressure control choke operates to automatically control and maintain a desired back pressure on the fluid passing through the choke. The choke housing has an inlet channel which intersects with an axial bore, part of which bore constitutes the discharge channel. An annular choke seat concentrically arranged in the discharge channel of the axial bore is adapted to receive an annular choke element. The choke element is affixed to a shuttle which is mounted on a fixed mandrel extending into the axial bore of the choke housing. The shuttle is dynamically sealed against the housing and mandrel such that a closed chamber is created in that portion of the axial bore opposite the discharge channel. The shuttle element is constructed to be fluid balanced such that it will move to achieve an equilibrium of pressure on its forward and rear surfaces. By introducing a desired pressure in the closed chamber, the shuttle and attached choke element will move and throttle the fluid flowing through the choke to automatically maintain a back pressure equal to the pressure in the closed chamber.

BACKGROUND OF THE INVENTION

The invention relates to a method and apparatus for controlling backpressure of fluid flowing through a choke device in a system, whereinthe coke automatically moves to maintain a desired back pressure on thefluid no matter what flow conditions exist or occur within the system.

During the drilling of oil wells wherein a circulating fluid such asdrilling mud is used, it is the practice at times to impose a backpressure on the drilling fluid. The back pressure, together with thehydrostatic pressure of the fluid in the well bore, contain the pressureof fluids within formations penetrated by the well bore and preventthese formation fluids from flowing into the well bore. Back pressurecontrol devices are also necessary for controlling "kicks," which arethe intrusion of salt water or formation gases into the drilling fluidwhich may lead to a blowout condition. In the case of a "kick,"sufficient additional back pressure must be imposed on the drillingfluid such that, in combination with the hydrostatic head pressure ofthe drilling fluid in the bore, the formation fluid is contained and thewell controlled until heavier fluid or mud can be circulated down thedrill string and up the annulus to kill the well.

It is also desirable to avoid the creation of "excessive" back pressureswhich could cause drill string to stick, or cause damage to theformation, the well casing, or the well head equipment.

Maintenance of a desired back pressure on the drilling fluid iscomplicated by variations in the flow characteristics which occur in thedrilling fluid passing through the back pressure control device. Whenflow characteristics of the drilling fluid entering the back pressurecontrol device change, e.g. the density of the fluid is altered by theinjection of debris or formation gases, or the volume of fluid enteringthe control device changes, the desired back pressure will not beachieved until appropriate changes have been made in the throttling ofthe drilling fluid in response to these changed flow conditions.Conventional devices generally required manual control of andadjustments to the choking device orifice to maintain the desiredbottom-hole pressures. However, manual control of the throttling deviceinvolves a lag time in making a correction and generally is inexact.Consequently, the art is in need of back pressure control devices whichare more automatic and accurate in response.

One approach to overcoming the foregoing described problems may be foundin U.S. Pat. No. 3,443,643, issued on May 13, 1969, to Jones. Thispatent discloses a choke having a "valve" element which carries anannular sleeve adapted to seat within an annular seal ring in a mannerwhich can throttle fluid entering the choke and passing through theannular seal ring. Movement of the ♭throttling" annular sleeve and valveelement is controlled by fluid pressures acting upon a piston connectedto the valve element. Well pressure conditions are monitored through acontrol device which automatically sends signals controlling pressureson the opposite faces of the "fluid balanced" piston, thereby causingappropriate movement of the valve element and throttling annular sleeve.While this patent discloses an "automatic" system, there is still aproblem of lag time between the identification of a change in flowconditions of the fluid entering the choke and the generation of signalsfrom the control monitor to the piston which controls the throttlingelement.

U.S. Pat. No. 4,044,834, issued Aug. 30, 1977 to Perkins discloses acontrol valve and system for a well bore designed to control the flow offluid from the well bore. This reference shows a device which respondsto changes in the pressure in the drilling fluid entering the device,but it is not a direct response such that the desired back pressure istruly automatically maintained. When conditions change suddenly suchthat well bore fluid pressure changes, pressure equalization, that is,the return to the actual desired back pressure, is not immediatelyobtained. A control system must sense the change which occurred in thewell bore pressure and generate a signal to a control piston which thenmoves the choke to achieve the desired back pressure on the drill fluid.

Other examples of pressure relief or a control devices are shown in U.S.Pat. Nos. 2,962,045, 3,778,022, and 4,167,262. However, none of thesedevices provide for an automatic response of the choking element to thesystem fluid such that the desired back pressure is maintained in thesystem.

SUMMARY OF THE INVENTION

The problems of the conventional choking devices are overcome by thefeatures and advantages of the present invention. The present inventionprovides a method and apparatus wherein a back pressure control chokeoperationally responds directly and automatically to changed fluidconditions without the need for control signals. A desired back pressurecan therefore be maintained on the fluid despite the changed flowconditions.

In accordance with the invention, the foregoing benefits have beenachieved through the present back pressure control choke apparatus. Theback pressure control choke of this invention has a housing whichincludes an inlet channel intersecting with an axial bore through thehousing. One end of the axial bore constitutes the discharge channel forthe fluid which is passed through the choke. A choke seat member isconcentrically mounted and supported within the axial bore in thedischarge channel at the intersection of the inlet channel with theaxial bore. A mandrel, fixed to the housing, coaxially extends into theaxial bore of the housing in that portion of the bore opposite thedischarge channel. A shuttle is slideably mounted about the mandrelwithin the axial bore.

The surface area of the forward axial end of the shuttle which faces thedischarge channel is substantially equal to the surface area of the rearaxial end of the shuttle. The shuttle is dynamically sealed against theaxial bore of the housing and against the mandrel such that a closedchamber is formed by the rear axial end of the shuttle, the mandrel, andthe housing, the chamber being in that portion of the axial boreopposite the discharge channel. A choke element is mounted to theforward axial end of the shuttle, concentric with the axial bore. Thischoke element has an outside cylindrical surface sized for insertioninto and withdrawal from the choke seat member thereby to control thefluid flow out the discharge channel.

A means is provided for applying a fluid pressure to the closed chamberand against the surface area of the rear axial end of the shuttle,thereby to control the movement of the substantially balanced shuttleand choke element relative to the choke seat member. Consequently, theback pressure exerted on the fluid passing through the choke isautomatically controlled in that the substantially balanced shuttle willrespond directly to changed conditions in the fluid flow to maintainequal pressures on its forward and rear surface areas.

A feature of the present invention includes a seal means carried by theshuttle and choke element whereby a positive seal stopping all flowthrough the housing is achieved when the choke element is fully insertedinto the choke seat. A further feature of the present invention residesin the use of a seal having the general shape of a right trianglewherein the right angle sides of the seal are abutted and retainedagainst the shuttle and choke element respectively in a manner such thatsubstantially complete insertion of the choke element into the chokeseat causes a positive seal of the leading edge of the choke seatagainst the hypotenuse side of the generally triangular seal. Theformation of this seal from a material such as Teflon, causes the sealto be "self-energizing" in that the Teflon will extrude as needed tomaintain a surface area which can seal against the leading edge of thechoke seat.

An additional feature of the present invention is that the choke elementand choke seat are reversible end-by-end. Yet another feature is thechamfered edges of the choke element and choke seat.

Still another feature of the present invention is the use of anindicator rod slideably extending through the mandrel, with one end ofthe rod extending outside the housing and the other end being connectedto the shuttle for corresponding movement therewith so as to provide anindication of the relative position of the choke element to the chokeseat and to monitor the wear of the choke element and choke seat.

A method for automatically controlling the back pressure on a flow of aprocess fluid through a choke device is also provided. The rear surfacearea of the slideable shuttle in the choke device is pressurized to alevel equal to the back pressure desired on the fluid flow in thesystem. The fluid flow is directed into the inlet channel, through thechoke device and to an outlet channel. A forward surface area of theslideable shuttle, equal to the pressurized rear surface area, issubjected to the fluid flow and the pressure generated therein. Inasmuchas the shuttle is pressure balanced, it will move in the direction ofthe lower pressure. By affixing a choke element to the slideable shuttlein a manner that movement of the shuttle will cause the choke element toreciprocate in and out of the path of the fluid flow, the pressure onthe fluid flow can be directly affected and controlled. The fluid flowis automatically choked by the choke element to create a back pressureon the fluid and on the forward surface area of the shuttle equal to thepressure being exerted on the rear surface area of the shuttle.

Where restricting of the fluid flow is not enough, the generation of adesired back pressure can be achieved by the sealing of the chokeelement in a choke seat such that a positive cutoff of fluid flowthrough the choke device occurs. Pressure buildup in the fluid willresult until it slightly exceeds the desired level. The shuttle willthen be urged away from the fluid and will withdraw the choke elementfrom the sealing arrangement thus permitting flow to resume at thedesired back pressure.

The back pressure control choke of the present invention, when comparedwith prior chokes and regulators, is operationally advantageous in thata desired back pressure on fluid can be automatically maintained withoutthe need for generating any "control" signals. It also permits apositive cutoff of the fluid by virtue of its seal arrangement, withminimal effect on the capacity of the choke to reopen in response to anexcessive back pressure on the fluid. Further, the choke of thisinvention automatically maintains the correct or desired back pressurewhen the fluid pump, i.e. a rig mud pump, is started up or shut down,thereby eliminating the possibility of pressure surges that alwaysexists during these times with a conventional hydraulically controlledchoke. Therefore, the choke of this invention is highly accurate incontrolling back pressure at desired pressure values during shutting andopening of fluid flow through the choke, as well as during throttlingthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional features and advantages will become apparent from thefollowing detailed description including the following figures wherelike reference numerals designate like parts in which:

FIG. 1 is a cross sectional view taken through the center of the housingof one embodiment of the invention; and,

FIG. 2 is a cross sectional view taken through the center of the housingof the embodiment shown in FIG. 1 wherein the choke element is in aclosed position with respect to the choke seat.

While the invention has been described in connection with a preferredembodiment, it will be understood that it is not intended to limit theinvention to that embodiment and many modifications will be apparent topersons skilled in the art.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, a cross sectional view of the back pressure control choke 10of the present invention is shown. The housing 12 for the choke is of agenerally T-shaped arrangement which incorporates an inlet channel 14which intersects and is substantially normal to an axial bore 16. Fluid,such as drilling fluid which is returning from a well after circulationtherethrough, would pass through inlet channel 14 to discharge channel18 which is part of the axial bore 16. The housing may be provided witha flange 15 as a means for connection of the choke to a well fluidreturn line, while flange 19 may be provided as a means for connectingthe choke to a suitable downstream flow line.

As noted, one application for the apparatus of this invention occurs inthe drilling of oil wells. It is customary to suspend a drill pipe inthe well bore with a bit on the lower end thereof and as the bit isrotated, to circulate a drilling fluid, such as a drilling mud, downthrough the interior of the drill string, out through the bit, and upthe annulus of the well bore to the surface. The circulation of fluid ismaintained for the purpose of removing cuttings from the well bore, forcooling the bit and for maintaining hydrostatic pressure in the wellbore to control formation gases and prevent blowouts and the like. Inthose cases where the weight of the drilling mud is not sufficient tocontain the bottom hole pressure in the well, it becomes necessary toapply additional back pressure on the drilling mud at the surface tocompensate for the lack of hydrostatic head and thereby keep the wellunder control. It is for this purpose that a back pressure control chokeof the present invention would be introduced into the return flow linefor the drilling fluid.

In accordance with this invention, back pressure is maintained on thefluid in the return line by the action of an annular choke element 38relative to annular choke seat 50. Choke element 38 is attached to ashuttle 28 which slideably moves within axial bore 16 upon fixed mandrel26. The opening between inlet channel 14 and discharge channel 18 iscontrolled by movement of the choke element 38 toward and into chokeseat 50. When choke element 38 is in its maximum open position, the"throttling" area, that is the passage area from the inlet channel tothe discharge channel, is equal to the area of the discharge channel.Reduction of the "throttling" area is achieved by movement of the chokeelement 38 toward the choke seat 50, which action creates a backpressure on the fluid in the inlet channel and back through the wellbore. Inasmuch as the characteristics of the drilling fluid (such as thecirculating rate, density, or temperature) do not remain constant, theability to maintain a constant back pressure on the drilling fluid, andconsequently a constant bottom hole well pressure, requires a throttlingaction by the choke element. The arrangement of the present inventioncauses the shuttle 28 to respond automatically to changed flowcharacteristics in the drilling fluid such that the choke element 38 ismoved relative to choke seat 50 to maintain a constant back pressure.

Referring again to FIG. 1, housing 12 may be provided with an end plate20 secured to the main housing by bolts 22. Mandrel 26 is fixed to endplate 20 and extends therethrough into axial bore 16. End plate 20 maybe provided with annular seal grooves 21 and 23 and O-ring seals 25 and27 respectively, thereby sealing the end plate against the mandrel andhousing.

Mandrel 26 has a shuttle 28 mounted thereon within the axial bore.Shuttle 28 is an annular element, concentric with mandrel 26 and axialbore 16. The shuttle has an enlarged diameter toward its rear end whichcontains an annular groove 29 for carrying a "T" ring seal 31. Theenlarged portion of the shuttle diameter also has a recessed surfacearea 33 on its forward side which further includes an annular groove 35.Abutting the recessed surface area 33 of the shuttle is channel ring 39which carries O-ring 41. Channel ring 39 is secured in place by lockring 43 fitted into annular groove 35. As will be described in moredetail hereafter, "T" ring seal 31 is the primary seal separating aclosed chamber 37 at one end of the shuttle and the well pressureddrilling fluid at the other, while O-ring 41 serves as a wiper ring.Shuttle 28 is also dynamically sealed against the mandrel 26 by "T" ringseal 65.

The closed chamber 37 is formed by the rear surface of shuttle 28,housing 12, mandrel 26, and the inside surface of end plate 20. Controlfluid ports 24 are provided in end plate 20 as a means for injectingfluid into closed chamber 37 at a desired pressure level.

Arranged against the internal cylindrical surface area of the forwardend of annular shuttle 28, and extending beyond the forward end of theshuttle, is annular choke element 38. An O-ring seal 45 is carried inannular groove 47, as a seal between the shuttle and choke element. Alock ring 40 is engaged in groove 49 of the choke element, which grooveis located at the longitudinal midpoint of the choke element. A forwardrecess area 51 of the shuttle 28 threadably receives retainer 46 whichacts to secure bolster ring 44, wedge seal 42, seal flange 53, and snapring 40, and consequently choke element 38, against the shuttle 28.Accordingly, as the shuttle 28 is moved to the left as shown in FIG. 1,flow of fluid from inlet channel 14 to discharge channel 18 isrestricted until fluid flow is completely stopped when choke element 38is moved to the position shown in FIG. 2.

As will be noted from FIG. 1, the axial bore to the left of theintersection of the inlet channel has a counterbore 55. Positionedwithin this counterbore are annular throat liners 57, with the linersituated closest to the inlet channel serving as the choke seat 50. Thethroat liners 57 may be identical in size and configuration with chokeseat 50 such that these elements could be interchangeable.

Choke seat 50, and throat liners 57, have an enlarged outside diameterabout their midsection such that recessed areas 59 exist on the outsidediameter at the ends of the choke seat and liners. Choke seat 50 abutsagainst throat liners 57 and these elements are retained in counterbore55 by retainer ring 54 and snap ring 61. The choke seat 50 is sealedagainst the housing 12 by O-ring seal 52 which is carried in housinggroove 62.

Choke seat 50 and throat liners 57 are preferably made of relativelyhard material, or coated therewith, as for example, a coating oftungsten carbide or the like.

As depicted in the drawings, annular choke seat 50 has an insidecylindrical opening sized to admit or receive annular choke element 38with minimal gap therebetween permitting insertion and removal of chokeelement 38 without contact or friction between their surface areas.

In the preferred embodiment of the present invention, a means isprovided for indicating the position of choke element 38 relative tochoke seat 50 and this can conveniently take the form of an elongatedposition rod 56 which is arranged to slideably extend through an axialbore in mandrel 26. The axial bore of the mandrel may be provided withappropriate packing 64 to provide a fluid tight seal with rod 56 and yetpermit the rod to extend outside of housing 12, as shown in FIG. 1. Theleft end of rod 56 is connected to shuttle 28 by means of spider 60mounted on the end of rod 56, as shown. The outer radial ends of spider60 are arranged for engagement in appropriate notches provided in theinternal bore of shuttle 28. Hence, upon movement of shuttle 28 ineither axial direction, rod 56 will move therewith. By monitoring theright end of rod 56 which extends from housing 12, a determination canbe made not only as to the relative position of choke element 38 withrespect to choke seat 50, but also of the wear on the choke element andchoke seat.

Shuttle 28 is what may be operationally described as a fluid balancedmember. That is to say, the total surface area of the right axial end ofshuttle 28 is substantially equal to the left axial end surfaces whichare subjected to the high pressure fluid which is present upstream fromchoke seat 50. Hence, shuttle 28 will translate axially back and forthover mandrel 26 to balance pressures on each side thereof. For purposesof balance, the retainer 46, bolster ring 44 and wedge seal 42 form apart of the left (forward) axial end surface of the shuttle 28 and donot alter the "balance" of surface area exposed to fluid pressure. Inaddition, shuttle 28 and choke element 38 are substantially balanced onthe downstream or a low pressure side, except for the surface area equalto the cross sectional area of rod 56.

In the normal operation of controlling back pressure on a fluid, such asin a well, the range of pressures on the low pressure side of the choke38 or downstream from choke seat 50 will be on the order of 0 to 50 PSI.The pressure on the upstream portion of the choke or the high pressureside will be on the order of 0 to 10,000 PSI. Hence, the relativeimbalance caused by the cross sectional surface area of rod 56 will besubstantially inconsequential with result that shuttle 28 may bedescribed as a substantially balanced shuttle. Therefore, any fluidpressure applied to closed chamber 37 will cause shuttle 28 and chokeelement 38 to move to the left, thereby restricting the flow of fluidthrough housing 12 until a pressure of equal magnitude is present ininlet channel 14 and acting upon shuttle 28.

Inasmuch as the shuttle 28 is directly exposed to the drilling fluidpassing through the housing, it will respond directly and automaticallyto any changes in flow characteristics of the drill fluid which wouldresult in an altered back pressure. There is no requirement for thedetection of a changed flow condition and subsequent generation of asignal to a throttling device. The response in the present invention isautomatic in terms of time and the lack of a need for an externalcontrol in "reply" to the need for throttling action. For example, wherethe density of the drilling fluid suddenly changes, as where a "gasslug" or debris pass through the housing causing a sudden drop or risein pressure, the shuttle will immediately respond by moving to the leftor to the right to achieve a back pressure on the drilling fluid whichis in equilibrium with the pressure existing in the closed chamber 37.

In those situations where a significant pressure drop occurs in inletchannel 14, or where a shut down of the circulation of the drillingfluid is desired, the apparatus of this invention achieves a positivecut-off of all flow. Referring to FIG. 2, a complete closure of thechoke of this invention is depicted. Choke element 38 is inserted withinchoke seat 50 until the leading edge of choke seat 50 contacts and sealsagainst wedge seal 42.

In the preferred embodiment, wedge seal 42 is a generally triangularlyshaped Teflon seal. This arrangement does not interfere with thethrottling of the choke and will withstand severe pressures and fluidvelocities therethrough. Furthermore, this arrangement permits apositive shut in without the disadvantage of a significant frictionfactor working against the automatic reopening of the choke, whichcondition may create momentary excessive back pressures and cause damageto the well operation.

Deformation of the wedge seal 42 during a positive shut in is minimizedby providing a seal flange 53 and bolster ring 44 which substantiallyencase the seal between choke element 38 and retainer 46. Sealing isachieved by the contact of the forward edge of choke seat 50 against thelower exposed portion of the "hypotenuse" side of wedge seal 42. Once aseal is achieved, this arrangement requires only approximately 6 PSI tobreak the seal. In view of the pressure being carried or exerted on thedrilling fluid in the inlet channel 14, this "break out" pressure of 6PSI is insignificant and permits the choke of this invention to remainvery accurate in maintaining a desired back pressure on the drillingfluid. The wedge-shaped geometry of seal 42 also increases theefficiency of the seal wherein the choke seat will continue to achieve apositive, fluid tight shut in. Furthermore, use of Teflon for the wedgeseal makes the seal "self-energizing" in that it will extrude asnecessary between choke element 38 and bolster ring 44 to provide asealing surface for contact with choke seat 50.

The means for applying fluid pressure (not shown in the drawing) toclosed chamber 37 will be apparent to those skilled in this art. Forexample, a hydraulic system, or a gas system utilizing a regulated gassupply could be utilized to communicate with control fluid ports 24 tothereby introduce and maintain the desired pressure to closed chamber37.

Additional efficiency and economy is achieved in the preferredembodiment of this invention by constructing choke element 38 and chokeseat 50 symmetrically such that they may be reversed end-by-endrespectively when their first ends have become too worn. Furthermore,the edges of choke element 38 and choke seat 50 may be chamfered toreduce stresses at those points caused by the high pressure fluidpassing thereby.

As previously described, T ring seal 31 serves as the primary sealbetween closed chamber 37 and the drilling fluid passing through thechoke of this invention. O-ring 41 serves as a wiper ring to removeabrasive particles of the drilling fluid from the inside cylindricalsurface of the housing which the T ring seal 31 also contacts, therebyserving as a means of protecting T ring seal 31 from abrasive wear.

In the preferred embodiment, channel ring 39, which carries O-ring 41,slides with shuttle 28 and is mounted thereon with a loose fit. Channelring 39 is not as wide as the space provided for it on recessed surfacearea 33 behind lock ring 43, and therefore is allowed to slip back andforth approximately 0.015 inches to 0.025 inches. As the shuttle slidesleft to right and right to left, the O-ring 41 does also, but not quiteas much due to the 0.015 to 0.025 inch "deadband" slip area. The insidediameter of channel ring 39 is also grooved axially to provide fluidcommunication between its left and right sides.

The deadband space permits the shuttle 28 to oscillate within that spacewithout necessitating the breaking of the static friction of the O-ring41 against the housing. This arrangement permits more precise trottlingcontrol with less friction drag from the O-ring 41. The deadband actionalso causes a pumping action in the cavity created between T ring seal31 and O-ring 41, thereby assuring no pressure buildup between the sealsand the evacuation of any contaminated mud or abrasive particles fromthis cavity.

The foregoing description of the invention has been directed in primarypart to a particular preferred embodiment in accordance with therequirements of the Patent Statutes and for purposes of explanation andillustration. It will be apparent, however, to those skilled in this artthat many modifications and changes in this specific apparatus may bemade without departing from the scope and spirit of the invention. Forexample, equivalent elements or materials may be substituted for thoseillustrated and described herein, parts may be reversed, and certainfeatures of the invention may be utilized independently of the use ofother features. It is applicants intention in the following claims tocover such modifications and variations as fall within the true spiritand scope of the invention.

What is claimed is:
 1. A back pressure control choke comprising:ahousing having an inlet channel intersecting with an axial bore, withone end of the axial bore defining a discharge channel for fluid passingthrough the choke; a choke seat member concentrically mounted within theaxial bore between the inlet channel and the discharge channel, thechoke seat member having a generally cylindrical internal surface; amandrel, fixed to the housing, coaxially extending into the axial borein that portion of the axial bore opposite the discharge channel; ashuttle, slideably mounted on the mandrel within the axial bore, thesurface area of the forward axial end of the shuttle facing thedischarge channel being substantially equal to the surface area of therear axial end of the shuttle; a first dynamic seal means between theshuttle and the axial bore of the housing; a second dynamic seal meansbetween the shuttle and the mandrel such that a closed chamber is formedwithin the axial bore by the rear of the shuttle, mandrel and housing; achoke element mounted to the forward end of the shuttle, cylindricallyshaped and concentric with the axial bore, the choke element having apredetermined size permitting insertion and withdrawal from the chokeseat member; and a means for applying fluid pressure to the closedchamber and the rear axial end of the shuttle, thereby controlling themovement of the shuttle and choke element relative to the choke seatmember and the back pressure exerted on the fluid passing through thechoke.
 2. The back pressure control choke of claim 1, further comprisinga seal means carried by the shuttle and choke element for sealingbetween the choke seat member and choke element when the choke elementis fully inserted into the choke seat member.
 3. The back pressurecontrol choke of claim 2, wherein the seal means comprises atriangularly shaped seal abutted and retained against the shuttle andchoke element in a manner that when substantially complete insertion ofthe choke element into the choke seat member occurs, the leading edge ofthe choke seat member will contact the seal and stop the flow of fluidthrough the choke.
 4. The back pressure control choke of claim 3,wherein the triangularly shaped seal is made of Teflon.
 5. The backpressure control choke of claim 2, wherein the seal means comprises aseal of extrudable material abutted and retained against the shuttle andchoke element in a manner that when substantially complete insertion ofthe choke element into the choke seat member occurs the seal will be insealing contact with the choke seat member.
 6. The back pressure controlchoke of claim 1, wherein the first dynamic seal means is comprised ofaT ring seal mounted in a groove about the outside circumference of theshuttle; a channel ring mounted about the outside circumference of theshuttle between the T ring seal and the fluid flowing through thecontrol choke; and a wiper ring mounted in the channel ring.
 7. The backpressure control choke of claim 6, wherein the channel ring has axialgrooves on its inside diameter to permit fluid communication between itsright and left sides and is mounted about the shuttle in a space of awidth greater than the channel ring.
 8. The back pressure control chokeof claim 1, wherein the choke element is reversible.
 9. The backpressure control choke of claim 1, wherein the choke seat member isreversible.
 10. The back pressure control choke of claim 1, wherein thechoke element and choke seat member have chamfered edges.
 11. The backpressure control choke of claim 1, further comprising a rod slideablyextending through the mandrel, with one end extending outside thehousing and the other end having means for connecting to the shuttle foraxial movement therewith, whereby the position of the outward end of therod is indicative of the position of the shuttle and choke element inthe housing.
 12. A back pressure control choke, for controlling backpressure of fluid flowing through the control choke, comprising:ahousing having an inlet channel intersecting with an axial bore, withone end of the axial bore defining a discharge channel for fluid passingthrough the choke; an annular choke seat member concentrically mountedwithin the axial bore between the inlet channel and the dischargechannel, the choke seat member having a generally cylindrical internalsurface; a mandrel, fixed to the housing, coaxially extending into theaxial bore in that portion of the axial bore opposite the dischargechannel; an annular shuttle, slideably mounted on the mandrel within theaxial bore, the surface area of the forward axial end of the shuttlefacing the discharge channel being substantially equal to the surfacearea of the rear axial end of the shuttle; a first dynamic seal meansbetween the shuttle and the axial bore of the housing; a second dynamicseal means between the shuttle and the mandrel such that a closedchamber is formed within the axial bore by the rear of the shuttle,mandrel and housing; an annular choke element mounted to the forward endof the shuttle, cylindrically shaped and concentric with the axial bore,the annular choke element having a predetermined size permittinginsertion and withdrawal from the annular choke seat member; atriangularly shaped seal abutted and retained against the shuttle andannular choke element in a manner that when substantially completeinsertion of the annular choke element into the annular choke seatoccurs, the leading edge of the annular choke seat contacts the seal andstops the flow of fluid through the choke; a rod slideably extendingthrough the mandrel, with one end of the rod extending outside thehousing and the other end having means for connecting to the shuttle foraxial movement therewith, whereby the position of the outward end of therod is indicative of the position of the shuttle and annular chokeelement in the housing; and a means for applying fluid pressure to theclosed chamber and the rear axial end of the shuttle thereby controllingthe movement of the shuttle and choke element relative to the choke seatmember and the back pressure exerted on the fluid passing through thechoke.
 13. The back pressure control choke of claim 12, wherein thetriangularly shaped material is made of Teflon.
 14. The back pressurecontrol choke of claim 12, wherein the triangularly shaped seal is madeof an extrudable material.
 15. The back pressure control choke of claim12, wherein the first dynamic seal means is comprised ofa T ring sealmounted in a groove about the outside circumference of the shuttle; achannel ring mounted about the outside circumference of the shuttlebetween the T ring seal and the fluid flowing through the control choke;and a wiper ring mounted in the channel ring.
 16. The back pressurecontrol choke of claim 15, wherein the channel ring has axial grooves onits inside diameter to permit fluid communication between its right andleft sides and is mounted about the shuttle in a space of a widthgreater than the channel ring.